Many plastic packages, such as those made from poly(ethylene terephthalate) (PET) and used in beverage containers, are formed by reheat blow-molding, or other operations that require heat softening of the polymer.
In reheat blow-molding, bottle preforms, which are test-tube shaped extrusion moldings, are heated above the glass transition temperature of the polymer, and then positioned in a bottle mold to receive pressurized air through their open end. This technology is well known in the art, as shown, for example in U.S. Pat. No. 3,733,309, incorporated herein by reference. In a typical blow-molding operation, radiation energy from quartz infrared heaters is generally used to reheat the preforms.
In the preparation of packaging containers using operations that require heat softening of the polymer, the reheat time, or the time required for the preform to reach the proper temperature for stretch blow molding (also called the heat-up time), affects both the productivity and the energy required. As processing equipment has improved, it has become possible to produce more units per unit time. Thus it is desirable to provide polyester compositions which provide improved reheat properties, by reheating faster (increased reheat rate), or with less reheat energy (increased reheat efficiency), or both, compared to conventional polyester compositions.
The aforementioned reheat properties vary with the absorption characteristics of the polymer itself. Heat lamps used for reheating polymer preforms are typically infrared heaters, such as quartz infrared lamps, having a broad light emission spectrum, with wavelengths ranging from about 500 nm to greater than 1,500 nm. However, polyesters, especially PET, absorb poorly in the region from 500 nm to 1,500 nm. Thus in order to maximize energy absorption from the lamps and increase the preform's reheat rate, materials that will increase infrared energy absorption are sometimes added to PET. Unfortunately, these materials tend to have a negative effect on the visual appearance of PET containers, for example increasing the haze level and/or causing the article to have a dark appearance. Further, since compounds with absorbance in the range of 400-700 nm appear colored to the human eye, materials that absorb in this wavelength range will impart color to the polymer.
A variety of black and gray body absorbing compounds have been used as reheat agents to improve the reheat characteristics of polyester preforms under reheat lamps. These reheat additives include carbon black, graphite, antimony metal, black iron oxide, red iron oxide, inert iron compounds, spinel pigments, and infrared absorbing dyes. The amount of absorbing compound that can be added to a polymer is limited by its impact on the visual properties of the polymer, such as brightness, which may be expressed as an L* value, and color, which is measured and expressed as an a* value and a b* value, as further described below.
To retain an acceptable level of brightness and color in the preform and resulting blown articles, the quantity of reheat additive may be decreased, which in turn decreases reheat rates. Thus, the type and amount of reheat additive added to a polyester resin is adjusted to strike the desired balance between increasing the reheat rate and retaining acceptable brightness and color levels. It would be ideal to simultaneously increase the reheat rate and decrease the rate at which color and brightness degrade as the concentration of the reheat additive in a thermoplastic composition is increased.
Another disadvantage of some conventional reheat additives known in the art is their instability during the PET manufacturing process. For example, antimony metal is known to re-oxidize to antimony oxide (which is ineffective at increasing reheat rate) if there are oxygen leaks in the melt-phase or solid-stating manufacturing processes. This results in variability in the heat-up rates of preforms in the reheat blow molding process and thus requires constant adjustments of the infrared lamp settings.
There remains a need in the art for polyester compositions containing reheat additives that improve reheat without the problems associated with known reheat additives, such as re-oxidation, inconsistent reheat, and unacceptable reductions in brightness, clarity, and color.